This invention relates to a DC-DC converter, in particular of the type capable of reducing the output electric current upon impedance drop of a load resulted from for example a short-circuited condition of the load.
A prior art DC-DC converter typically includes a transformer which has a primary winding connected in series to a DC power source and a switching transistor, and a secondary winding connected through a smoothing circuit to output terminals for applying an output voltage on a load. A control circuit produces output pulses to the switching transistor which is thereby intermittently turned ON and OFF to generate electric current from the DC power source through the primary winding of the transformer and switching transistor. Electric current through the primary winding provides the transformer with electric energy to be converted into electric current through the secondary winding so that the electric current flows from the secondary winding through the smoothing circuit and output terminals. The secondary winding and smoothing circuit produce the output voltage of the level different from that of the DC power source due to a turn ratio of the primary to secondary winding. Also, the output voltage can be adjusted to an appropriate level by controlling the pulse width of ON signals supplied to the base or gate terminal of the switching transistor from the control circuit. If the output voltage is increased to a higher level than a regular level, the control circuit produces output pulses of narrower time width to the control terminal of the switching transistor to reduce the ON period of time for the switching transistor. Adversely, if the output voltage is reduced to a lower level than the regular level, the control circuit produces output pulses of wider time width to the control terminal of the switching transistor to expand the ON period of time for the switching transistor.
In most cases, recent DC-DC converters require reduced electric power consumption in a short-circuited condition of the load as well as during the regular operative condition of the load. Each of these DC-DC converters includes the control circuit which comprises a hybrid integrated circuit called as xe2x80x9cBi-CMOSxe2x80x9d of a bipolar element and a CMOS (Complimentary Metal Oxide Semiconductor) element to reduce the consumed electric power in the control circuit. However, when the load impedance remarkably drops due to for example the short-circuited condition of the load, the output electric current uncontrollably increases to thereby cause damage to or destroy of the smoothing circuit through which the increased electric current flows.
An object of the present invention is to provide a DC-DC converter capable of reducing the output electric current upon impedance drop of a load resulted from for example a short-circuited condition of the load. Another object of the invention is to provide a DC-DC converter which can prevent damage to or destroy of an electric element through which the output electric current flows.
The DC-DC converter according to the present invention comprises a transformer (2) having a primary winding (2a), secondary winding (2b) and a drive winding (2c); a switching element (3) connected in series to a DC power source (1) and the primary winding (2a); a first output smoothing circuit (4) connected to the secondary winding (2b) and output terminals; and a control circuit (10) for generating outputs to drive the switching element (3). When voltage of a trigger level (VON) is applied on the control circuit (10), it is operable to produce output pulses in order to alternately turn the switching element (3) ON and OFF for generation of the stabilized output from the output terminals through the first output smoothing circuit (4), however, the control circuit (10) is inoperable when voltage of an inoperative level (VOFF) is applied on the control circuit (10). The DC-DC converter further comprises a trigger circuit (11) for supplying electric power from the power source (1) to the control circuit (10) at the beginning of operating the control circuit (10); a second output smoothing circuit (7) connected to the drive winding (2c) for supplying electric power to the control circuit (10); and a voltage control circuit (26) for detecting the output voltage (Vcc) of the drive winding (2c) under the trigger level (VON) for driving the control circuit (10).
When the load impedance is remarkably lowered by such as a short-circuited condition which may happen in a load (25), the voltage control circuit (26) can be operated to forcibly decrease the output voltage of the second output smoothing circuit (7) to the inoperative level (VOFF) for ceasing operation of the control circuit (10) when the second output smoothing circuit (7) is electrically charged to the trigger level (VON) through the trigger circuit (11). The forcible decreasing of the output voltage of the second output smoothing circuit (7) is very effective to shorten time width of the ON output voltage (VO) during the intermittently oscillating operation of the converter, thus reducing the output current.
In another embodiment of the invention, the transformer (2) may have a detective winding (2d); a second output smoothing circuit (34) connected to the detective winding (2d); and a voltage control circuit (26) for detecting output voltage of the detective winding (2d) under a trigger level (VON) for driving the control circuit (10). In a further embodiment of the invention, a transformer (2) has an autotransformer (37) with a primary winding (2a), a drive winding (2c), a tap (37b) provided in the primary winding (2a) of the autotransformer (37) and a first output smoothing circuit (4) connected between an end and the tap (37b) of the primary winding (2a) and to output terminals.
The voltage control circuit (26) is operable to increase the output voltage (Vcc) of the second output smoothing circuit (7) when the load impedance is restored to a normal level so that the drive winding (2c) produces the voltage above the trigger level (VON). The second output smoothing circuit (7) comprises a capacitor (9) connected to the control circuit (10). The voltage control circuit (26) comprises a threshold circuit (28) for detecting the voltage of the drive winding (2c) under the trigger level (VON); and a deactivating circuit (30 to 33) for discharging the electric charge accumulated in the capacitor (9) to the inoperative level (VOFF) for ceasing operation of the control circuit (10) when the threshold circuit (28) detects the voltage of the drive winding (2c) under the trigger level (VON). The deactivating circuit (30 to 33) comprises a switching circuit (31, 32) connected in parallel to the capacitor (9) for discharging the electric charge in the capacitor (9) to the inoperative level (VOFF) for ceasing operation of the control circuit (10) when the threshold circuit (28) detects the voltage of the additional winding (2c, 2d) under the trigger level (VON). Another embodiment of the instant invention may comprise a transformer (2) of an autotransformer (37) with a detective winding (2d).